Ionic flame monitor

ABSTRACT

A protection circuit for an ionic flame monitor to insure that high voltage AC signals appearing across the flame electrodes and arising from sources other than a flame will not be effective to provide a false output indication of flame presence. The flame monitor is sensitive to AC signals above a certain frequency or above a certain voltage to indicate flame presence. Non-flame AC signals having voltages above or below the certain frequency, but above the certain voltage are prevented from indicating flame presence if their voltage is above a second certain level which is above that of most flame signal voltages. A voltage level detection circuit clamps the monitor output circuitry to a &#39;&#39;&#39;&#39;no-flame&#39;&#39;&#39;&#39; condition whenever the AC signal voltage is above the second certain level.

United States Patent 1 Taylor June 19, 1973 IONIC FLAME MONITOR [75]Inventor: Jonathan Todd Taylor, Simsbury,

Conn.

[73] Assignee: Combustion Engineering, Inc.,

Windsor, Conn [22] Filed: Dec. 30, 1971 [21] Appl. No.: 214,129

[52] US. Cl. 307/117, 340/228.2 [51] Int. Cl. G08b 21/00 [58] Field ofSearch 307/117; 328/6;

[56] References Cited UNITED STATES PATENTS 2,766,440 l0/1956 Marsden340/228 R Mandock et a1 307/117 X Primary Examinerl-lerman J. HohauserAssistant Examiner-M. Ginsburg Attorney-Eldon H. Luther and Stephen A.Schneeberger [57] ABSTRACT A protection circuit for an ionic flamemonitor to insure that high voltage AC signals appearing across the vflame electrodes and arising from sources other than a flame will not beeffective to provide a false output indication of flame presence. Theflame monitor is sensitive to AC signals above a certain frequency orabove a certain voltage to indicate flame presence. Non-flame AC signalshaving voltages above or below the certain frequency, but above thecertain voltage are prevented from indicating flame presence if theirvoltage is above a second certain level which is above that of mostflame signal voltagesv A voltage level detection circuit clamps themonitor output circuitry to a no-flame condition whenever the AC signalvoltage is above the second certain level.

7 Claims, 4 Drawing Figures Patented June 19, 1973 2 Shoots-Sheet IIONIC FLAME MONITOR BACKGROUND OF THE INVENTION The invention relates toflame monitors and more particularly to flame monitors of a type whichuse the principle of flame ionization for determining the presence orabsence of a flame.

In many applications, including both home and industry, flame combustionof fuels is used as a source of heat. It is essential in most suchapplications, in the interests of safety and economy, that there be atno time a sizable accumulation of unburned fuel in the combustion zone(such as might occur upon flame failure or initial failure of the fuelto ignite). Such unburned fuels may be ignited due to spurious causesand precipitate an explosion. There are available today various types offlame monitors or detectors for indicating flame failure and preventingthe resulting buildup of the potentially hazardous condition. Theseflame monitors may be light sensitive devices or they may respond todifferential pressures within a combustion chamber or they may rely uponthe electrical properties of the flame to provide an indication of flamepresence or absence.

The latter mentioned technique of flame monitoring has variousadvantages which have lead to its increased usage. More particularly,flame monitors have been developed which exploit the fact that anionizing process occurs within the flame due to the fuel combustion. Inthe combustion process, excess energy is liberated by the combining oftwo or more elements to form a com-- pound with a lower potential energylevel. Ions, taking the form of electrons and positive atom nuclei, areformed by the heat of the combustion process. High speed photography hasshown that most flames do not burn in a continuous uniform manner, butrather as many tiny discreet packets, the sum total of which form theflame seen by the eye. In each of these burning packets is a collectionof ions having positive and negative potentials.

If an electrical potential, preferably a DC potential, is placed acrossthe flame, an AC current will be generated. The AC current so generatedmay be used to develop a flame signal which is subsequently used toindicate the presence or absence of flame. An early example of thisutilization of the AC voltage fluctuations generated by the action of aflame between a pair of spaced electrodes will be found in U.S. Pat. No.

2,766,440 issued Oct. 9, 1956 to R. S. Marsden.

More recently, ionic flame monitors have been developed which exploitthe fact that the AC voltage fluctuations generated by the flame arerich in frequencies within a particular range. Signal analysis hasrevealed that the flame generated AC signal is particularly rich infrequencies within the range from 200 Hz to 2,000 Hz. Thischaracteristic of the flame generated AC signal has permitted thedevelopment of flame monitors which attempt to recognize only aparticular range of flame signal frequencies as being indicative offlame presence, andthereby preclude the possibility of a false externalsignal at a different frequency, for instance 60 Hz line voltage, beingsensed as an indication of flame presence. This type of ionic flamedetector typically uses some form of frequency discriminator connectedintermediate the flame electrodes and the flame switch in the outputcircuitry of the monitor to pass, to the switch, flame signal energywithin the range of frequencies most common in a flame and to reject orstrongly attenuate other signal frequencies, such as stray 60 Hzcurrents.

However, these frequency discriminators are often effective for thispurpose only within a limited range of input voltages and may passsufficient signal energy to indicate flame presence when the strength ofthe signal applied to the discriminator is of extremely high magnitude,even though the frequency of the applied signal may be within the normalrejection or attenuation range. This problem may arise with flamemonitors op erating in a region where 60 Hz line voltage and secondharmonic components thereof are present. Also, instances may occur inwhich AC voltages appear at the input to the frequency discriminator andhave frequency characteristics similar to those generated by a flame butare of a larger amplitude than the flame signal generated by a flame.These false signals may arise due to high voltage, high frequency noiseassociated with ignitor spark plugs and they may also arise through 60Hz line voltage appearing across the flame elec trodes because ofelectrical leakage or a direct short in the line supply system. In thesesuch instances the fre quency discriminator passes false triggeringenergy to the flame switch because the input signal amplitude is aboveits rejection capabilities. When this occurs, the flame monitor outputcircuitry will respond in a manner commensurate with flame presence,when in fact there may be no flame present. Obviously, such anoccurrence may have dangerous consequences.

therefor, it is an object of the present invention to provide anionicflame monitor which will accurately indicate and/or respond only toan AC signal of the frequency and magnitude actually generated by aflame and will indicate flame absence at all other times including thosein which high voltage signals from sources other than a flame are passedby the frequency discriminator.

SUMMARY OF THE INVENTION According to the invention, there is provided aflame monitor for determining the presence or absence of a flame withina flame zone through detection of a flame generated AC voltage. A pairof electrodes or flame rods are spaced apart from oneanother anddisposed within the flame zone such that they are in contact with theflame when it is present. A source of DC potential is impressed acrossthe flame electrodes for increasing the rate of ion migration to theoppositely poled electrodes, thus resulting in a flame generated signalof significant strength. Flame signal processing means are operativelyconnected across the electrodes to sense any AC signals appearingthereacross and have circuit characteristics which process the signalsin a manner which normally provides an output response indicative offlame presence whenever the AC signal sensed is substantially anyvoltage within a particular frequency range characteristic of a flamegenerated signal or is above some particular AC voltage level in afrequency range other than that characteristic of a flame.

In order to prevent the flame signal processing circuitry from providingan output response indicative of flame presence in those instances inwhich the AC signal at the input thereto arises from sources other thanthe flame, means are provided to effect an output response indicative offlame absence whenever the AC voltage across the electrodes exceeds apredetermined peak value. This peak value is selected such that it isslightly greater than the highest AC peak voltages normally generated inthe actual flame signal, thus permitting that output responsecommensurate with flame presence only when the AC signal across theelectrodes is within both the frequency and voltage rangescharacteristic of a flame generated signal.

The signal processing mean preferably include a flame switch and signalcoupling means, said flame switch being responsive to signal energypassed from the electrodes by said coupling means for controlling theoutput response indicative of the flame presence or absence. Thecoupling means preferably are frequency discriminating and may include afilter which has highpass characteristics. A clamp is applies to theflame switch circuit to effect the flame absent response whenever the ACvoltage across the electrodes exceeds the above mentioned predeterminedparticular peak value.

A discharge device having a break-down potential at said particularvalue of peak voltage is connected across said electrodes. When the ACvoltage exceeds this level, the discharge device conducts, providing avoltage drop across an impedance means in series therewith. In thepreferred embodiment of the invention, this voltage drop is utilized toactuate a switch and close a low impedance circuit between the flameswitch input'circuit and ground. This low impedance circuit to groundclamps the flame switch input and prevents it from responding to signalenergy in the manner required to indicate flame presence. The flameswitch, having its input circuit clamped to ground potential, effects anoutput response indicative of flame absence, thus insuring that only anAC signal as generated by a flame, will be able to effect the flamepresent re sponse of the flame switch.

DESCRIPTION OF THE DRAWINGS tection circuit-of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown a functional block diagram of the ionic flame monitor of theinvention. A flame issues from some type of burner, for instance ignitor12. An electrode, such as flame rod 14, is spaced from ignitor l2 andpositioned such that it is contacted by the flame 10 in a zone occupiedby the As earlier described, the ion packets within a flame 10 createpotential variations between electrodes 12 and 14 giving rise to an ACcurrent therebetween. The amplitude of this current is enhanced by theDC potential impressed across the flame 10 between electrodes 12 and 14and a flame signal may be derived from the AC voltage fluctuationsappearing across resistor 22 due to the flame generated AC current inthe circuit including resistor 22 and the electrodes.

The AC flame signal developed across resistor 22 is connected to flamesignal processing means 24 through conductor 26. Conductor 26 isconnected at one end to conductor 20 intermediate resistor 22 and flamerod 14 and is connected at its other end to the input of signalprocessing means 24. The signal energy appearing at conductor 26 willcomprise, when flame 10 is present, an AC signal as generated by theflame, superimposed on a DC potential provided by supply 16. Signalprocessing means 24 include coupling means 28 and flame switch means 30.The input circuit of signal processing means 24 is also the input tocoupling means 28. Coupling means 28 typically serves to separate theflame when a normal flame condition exists. Ignitor l2 AC component ofthe signal from the DC component and to pass to the output thereofsignal energy having a magnitude determined by the AC frequency of thesignal applied at the input thereto. This signal energy appearing at theoutput of coupling means 28 is connected to the input circuit of flameswitch means 30 by conductor means 32.

Flame switch means 30 comprise circuit means responsive to the magnitudeof the signal energy connected thereto by conductor means 32 to providea first output response when the signal energy magnitude is equal to orgreater than a certain value and to provide a second output responsewhen the signal energy magnitude is below said certain value. Theresponse of flame switch means 30 may be electrical or electromagneticin nature and typically serves to place utilization means 34 in one oftwo states. Utilization means 34 typically includes a two state loaddevice 36 such as a lamp, a fuel control valve or the like, a powersupply 38 and connecting circuit means including switch contacts 40. Thecircuit means serve to apply power to said load device 36 when switchcontacts 40 are closed and the power is interrupted when contacts 40 areopen.

The output response of flame switch 30 and accordingly of signalprocessor 24 is here represented by broken line 42 in FIG. 1 and may beindicative of an electromagnetic field created by flame switch means 30and operative to open or close contacts 40 associated with load device36. It is desirable to recognize only certain AC signal frequencies asbeing indicative of the existance or presence of flame 10. Therefore,the coupling means 28 have frequency discriminating characteristicswhich strongly attenuate signals having frequencies below thosecharacteristically present in a flame signal and passing, with onlyslight attenuation, signals having frequencies in the rangecharacteristic to a flame generated AC voltage. Thus, the signal energymagnitude at the output of coupling means 28, for an input signal of thesame magnitude, will be greater when the input signal is in the flamefrequency range than when it is below this frequency range. The largeroutput signal magnitude from coupling means 28 is utilized to effectsaid first output response from flame switch means 30 and, when themagnitude of the output signal is below a certain value, then saidsecond output response from switch means 30 is effected. Thus said firstoutput response from flame switch means 30 and accordingly flame signalprocessor 24 is commensurate with flame presence and said second outputresponse is commensurate with flame absence.

Under normal circumstances when a flame is present, an AC signal offairly uniform magnitude will appear at the input to coupling means 28resulting in an output signal therefrom of fairly uniform magnitude. Thesensitivity of the input circuit to flame switch means 30 may beinitially adjusted to effect that output response indicative of flamepresence for input signals of this magnitude and greater and to effectthat output response indicative of flame absence when the input signalmagnitude is below this value. However, the nature of coupling means 28are often such that when the voltage amplitude of the input signalthereto is increased the magnitude of the signal output therefrom isalso increased regardless of frequencies. Thus, an AC signal appearingon conductor 26 and having a voltage significantly above that of anormal flame signal may provide a signal of large enough magnitude atthe output of coupling means 28 to effect the flame present responsefrom switch means 30 even though it is at a lower frequency thananactual flame signal. This con dition might arise through a short of theAC line supply across electrodes 12 and 14 or it may also occur as theresult of high voltage arcing or discharges occurring in close proximityto the electrodes and associated circuitry. If a flame 10 is not presentwhen these signals occur, an output response from switch means 30commensurate with flame presence could have dangerous consequencesparticularly if load device 36 is a fuel control valve. I

In order to avoid the consequences which might arise due to an outputresponse from flame switch 30 which inaccurately indicated flamepresence, circuit means are provided to prevent the flame present"response whenever the AC voltage appearing across electrodes 12 and 14exceeds a particular value. The particular value chosen is such that itis slightly greater than substantially any AC voltage which would begenerated by flame 10. Accordingly, clamping means 44 are operativelyconnected across electrodes 12 and 14 to sense the amplitude of ACvoltage appearing thereacross and responds in a manner which clamps theinput circuit to flame switch 30 whenever the AC voltage appearingacross electrodes 12 and 14 exceeds the particular value. Typically, theinput circuit to flame switch 30 is clamped. by connecting it through alow impedance circuit to ground potential thus preventing the outputresponse commensurate with flames presence and rather providing thatresponse commensurate with flame absence. v

Clamping means 44 include voltage level detection means 46 and aclamping switch 48. Clamping switch 48 includes means for responding toa switching voltage to open or to close a circuit between groundpotential and the input circuit'to flame switch 30. Voltage leveldetection means 46 are connected across electrodes l2 and 14 to sensethe AC voltage appearing thereacross and to provide said switchingvoltage to close clamp switch 48 when the AC voltage sensed is greaterthan said particular value referred to above.

A more thorough understanding of the invention may be derived from adetailed description of the circuitry of a preferred embodiment of theinvention as shown in flG. 2. ignitor 12 is electrically connected toground and is the source for a flame 10. Flame rod 14 is spaced fromignitor 12 and is positioned to be in contact with flame 10 under normalflame conditions.

A transformer 50 having its primary connected to a source of 60 Hz 120Vline current is used to provide the necessary power to the flamemonitor. A secondary coil 52 is center tapped to ground and diodes 54and 56 connected to opposite ends thereof in parallel provide a fullwave rectified DC voltage at the junction 58. Resistor 60 and capacitor62 are connected in series between junction 58 and ground and serve tolimit and filter the voltage appearing at junction 58. A second resistor64 is connected in parallel with capacitor 62 to ground and acts as ableeder to discharge the capacitor when AC power to the circuit isremoved. A resistor 22 is connected at one end to the junction betweenresistor 60 and capacitor 62 to receive a filtered DC voltage of severalhundred volts, in this instance 500V DC. The other end of resistor 22 isconnected to conductor 20 which is in turn connected to flame rod 14,thus impressing a DC potential of some 500V across electrodes l2 and 14.Resistor 22 typically has a value of K ohms and is utilized to developthe flame signal. When flame 10 appears between and contacts electrodesl2 and 14, the flame generated AC current, described earlier, results.This current through resistor 22 provides a voltage drop thereacrosswhich results in an AC flame signal voltage appearing at conductor 20between resistor 22 and flame rod 14. This AC flame signal is connectedto the input of flame processing means 24 by conductor means 26.

Conductor 26 applies the signal voltage resulting between electrodes 12and 14 to signal coupling circuit 28. In the preferred embodiment,coupling means 28 is comprised of a transformer 66 and capacitors 68 and70 connected to form a combination double tuned circuit and impedancematcher. The input circuit of coupling means 28 includes the primary oftransformer 66 connected in series with capacitor 68 and connectedacross electrodes 12 and 14 through conductor 26. The output circuit ofcoupling means 28 includes the secondary of transformer 66 in serieswith capacitor 70. One side of both the primary and secondary oftransformer 66 is connected to ground. The turns ratio of transformer 66primary to secondary is typically ten to one. Capacitor 68 is connectedto conductor 26 to receive the AC signals appearing across theelectrodes. The output from coupling means 28 appears at that plate ofcapacitor 70 remote from the secondary of transformer 66. In thepreferred embodiment, transformer 66 is a TA-47 manufactured by Stancorand capacitors 68 and 70 respectively have values of 0.01 pf and 0.33[.Lf.

The tuned impedance matching circuit of coupling means 28 serves tocouple the high impedance flame rod circuit to the low impedance inputof the circuitry coupled thereafter with maximum signal development andalso strongly attenuates signals below about 200 Hz to eliminate 60 andHz AC pickup. Because of the high input and low output impedancecharacteristics of coupling means 28, the circuit across flameelectrodes 12 and 14 is not heavily loaded, thus permitting thedevelopment of a strong flame signal at the input, and subsequently atthe output, of the coupling means. The signal energy at the input tocoupling means 28 appears as a high voltage, low current whereas theenergy at the output is of higher current and lower voltage.

The filtering or energy transfer characteristics of coupling means 28are seen graphically in FIG. 3 wherein the ratio of energy-out toenergy-in, measured along the vertical axis, is plotted againstfrequency measured along the horizontal axis. It will be noted that theoutput signal energy from coupling means 28 relative to the signal inputenergy thereto is only slightly attenuated for signal frequencies above200 Hz, but that for frequencies below 200 Hz and particularly below 150Hz the attenuation is great. It must, however, be noted that at aparticular frequency the output energy is a particular function of theinput energy as determined bythe filter characteristics, and anincreased input signal will result in an increased output signal eventhough at that frequency the output signal may be greatly attenuatedrelative to the input. As has earlier been mentioned, the AC signalgenerated by flame 10 is rich in frequencies of 200 Hz and above andcoupling means 28 have been tuned to pass this frequency range andattenuate lower frequency signals. Under normal flame conditions, an ACsignal-voltage of some 60V will be developed across the input tocoupling means 28 and will result in an output voltage of about 1V.However, in those instances in which a non-flame generated high voltageAC signal appears across the electrodes, it may also be coupled to theoutput circuitry through coupling means 28. This will be particularlytrue if its frequency is in the filter pass range, but a signal ofsubstantial magnitude may also appear at the output of the couplingmeans for input signals having frequencies in the 60 to 120 Hz range ifthe input voltage is sufficiently large.

In the preferred embodiment, flame switch means 30 are connected to alow voltage (26V) source of AC voltage provided by secondary 70 oftransformer 50. Flame switch means 30 include a load relay coil 72 inseries with gate means, such as silicon control-rectifier (SCR) 74,across the AC voltage of transformer secondary 70. Relay 72 operates toelectromagnetically actuate the contacts 40 in utilization circuit 34described above. Contacts 40 are open when relay 72 is not energized andare electromagnetically closed when the relay is energized. Relay 72 isenergized by current conduction therethrough when SCR 74 is in itstriggered conductive state.

SCR 74 includes a cathode 73 connected to ground,

an anode operatively connected to one end of relay coil 72 and a triggerelectrode 76. A trigger circuit, comprising the SCR trigger electrode 76and cathode 73, is

- in the input circuit to flame switch means 30 and is responsive to themagnitude of a signal, in this instance a triggering voltage, appliedthereto to initiate conduction by the SCR only when the signal is abovea certain magnitude. As an AC supply voltage is used, SCR 74 will ceaseto conduct after each cycle unless the enabling trigger signal ismaintained at trigger electrode 76. A capacitor 78, connected inparallel with relay coil 72, and a resistor 80, connected in series withsaid coil 72 and SCR 74 serve to delay the initial energization of therelay coil 72 by 0.25 second following triggering of the SCR. Further,capacitor 78 maintains the relay coil 72 energized for a short period,1.5 2 seconds, following termination of conduction by and through SCR74.

The trigger signal applied to trigger electrode 76 is a function of thesignal output from coupling means 28.

This trigger signal is developed across a resistor, such aspotentiometer 82, connected in the trigger circuit between triggerelectrode 76 and ground. In some instances, the magnitude of the signalenergy appearing at the output of coupling means 28 may be sufficient touse as the trigger signal applied to potentiometer 82 to provide thetrigger signal. However, this is not usually the case and it ispreferred that an amplifier 84 be interposed between the output couplingmeans 28 and the Y the output circuit of coupling means 28, or part ofthe input circuit to flame switch 30, or more generally, as merely asignal conductor between the two. A DC power source for amplifier 84 isprovided by means of rectifying diode 86 and fllter capacitor 88operatively connected across the secondary of transformer 50. A 40V DCpotential appears at the junction 90 between capacitor 88 and thecathode of diode 86. This DC potential is connected to amplifier 84. Theoutput of amplifier 84 is an AC voltage, the magnitude of which isproportional to the magnitude of the input signal thereto. Thisamplifier output signal is applied to the wiper arm of potentiometer 82.Through use of amplifier 84 and potentiometer 82, the AC signal energypassed by coupling means 28 is scaled such that it will provide atriggering voltage to the trigger electrode of SCR 74 when it is above acertain selected magnitude and will be below the triggering potentialwhen it is below the certain magnitude. The threshold magnitude at andabove which the signal output from coupling means 28 is intended totrigger SCR 74 into conduction is established as that which occurs for anormal flame signal voltage of about 45-60 peak volts AC acrosselectrodes 12 and 14 and being predominantly above about Hz infrequency. If it is desired that a flame signal of lesser magnitude becapable of triggering SCR 74, the sensitivity may be readily adjusted byvarying the setting of potentiometer 82. Potentiometer 82 typically hasa resistance value of 0-1 K ohm.

As earlier mentioned, coupling means 28 does not attenuate all inputsignals to the same energy levels for a particular frequency, but ratherattenuates them by some amount relative to input amplitude. Thus, inputsignals from sources other than a flame and having AC voltages greaterthan 60-80V, whether in the normal flame frequency range or below it,may be passed by the coupling means at a sufficient magnitude to effecttriggering of SCR 74, which energizes relay 72 causing a responsecommensurate with flame presence when in fact a flame may not bepresent.

To prevent a high voltage non-flame generated AC signal which mightappear across electrodes 12 and 14 from effecting an erroneous responseindicating flame presence, clamping means 44 are provided. The term highvoltage is meant to refer to AC voltages greater than those occurring ina normal AC flame signal. Clamping means 44 include a switch 48,responsive to a signal provided by AC voltage level detection means 46,to close a low-impedance circuit to ground across the input or triggercircuit of SCR 74 and accordingly, flame switch means 30. This lowimpedance circuit itconducts. The

serves to clamp the trigger electrode 76 to the ground potential ofcathode 73, thus preventing triggering of the SCR into conduction.

Voltage level detection means 46 comprise a circuit connected acrosselectrodes 12 and 14 including, in series, DC isolation means such ascapacitor 90, voltage threshold break-down means such as neon lamp 92and Zener diode 94, and impedance means, such as resistor 98, acrosswhich a switching voltage may be developed. One terminal of capacitor 90is connected to conductor 20 and the other is connected to the voltagethreshold break-down means. Resistor 98 is connected between the voltagethreshold break-down means and ground. Capacitor 90 will be charged tothe DC potential applied across the electrodes 12 and 14 and thus servesto isolate or prevent that DC potential from appearing across thevoltage threshold break-down means. With the DC potential thus isolated,the only potential appearing across Zener diode 94 and neon lamp 92 isthe AC voltage appearing between electrodes 12 and 14; This AC potentialis passed" by capacitor 90.

The voltage threshold break-down means are selected to break down andconduct when a potential difference greater than about 70 or 80 volts isimpressed across them. This potential is slightly above the 45-60 peakAC voltageappearing between electrodes 12 and l4of the aforedescribedflame monitor due to the pres ence of flame In the preferred embodiment,neon lamp 92'hasfa break-down potential of about 60V and Zener diode 94has a break-down voltage of about V. Zener diode 94 might be replacedwith a Diac. The Zener diode 94 is poled to conduct freely whenelectrode l4 is negative relative to ground and to break down andconduct at and above about 15V when the electrode is positive relativeto ground. The fact that Zener 94 conducts at a much lower voltage'than15V for'the former (or negative) polarity does not interfere withproperoperation of the clamping switch 48, as will become evident hereinafter.

The combination of neon lamp 92 and Zener diode 94 might be replacedwith a single Zener diode 95, as seen in FIG.'4, having a break-downvoltage of 70 to 80V. However economics suggest the use of a Zener orZeners with lower break-down voltages and further, the neon lamp 92serves to visually indicate the presence of the excessive AC voltageacross the electrodes when lamp may signal this condition to an When theZener diode and neon-lamp conduct, the current through resistor 98creates a voltage drop thereacross'. This voltage will be thatinstantaneous voltage appearing across electrodes 12 and 14 minus :thevoltage drop of about 70V across the Zener diode and neon lamp. Resistor98 has a resistance of 47 K ohml I The voltage developedacross resistor98 is utilized as a switching voltage for the clamping switch 48. Theclamping, switch includes a transistor 99 having its emitter 100connected to ground and its collector 102 connected to the input circuitof flame switch means 30. More particularly, collector 102 iselectrically connected by conductor 104 to the trigger electrode 76 ofSCR 74. Transistor 99 will be switched on,'or conduct, when the voltageat base 106 is a positive 0.6V. The switching voltage developed acrossresistor 98 is connected to the base 106 of transistor 99 through diode108 and current limiting resistor 110. The. anode of diode 108 isconnected to the junction of resistor 98 and the voltage thresholdbreak-down means, with its prevents discharging of capacitor 112 by theopposite (or negative) polarity of the AC voltage across electrodes l2and 14, and further, applies to base 106 only that polarity of voltagecapable of biasing the transistor 99 into conduction.

With the Zener diode 94 of FIG. 2 or Zener diode 95 of FIG. 4 poled asdescribed above and diode 108 connected as described hereinbefore, Zener94 or Zener 95 will be connected to permit a positive voltage on theanode of diode 108 only when the AC potential applied to the oppositeelectrodes of the Zener exceeds its break-down voltage. Though the Zenermay conduct in the opposite direction at much lower voltages, diode 108will then be reverse biased and will not conduct.

In operation, if an AC signal having a voltage greater than about 70peak volts appear across electrodes 12 and 14, it will cause breakdownand conduction by the voltage level detection means 44 and will switchtransistor 99 into conduction. When transistor 99 conducts, it serves toprovide a low impedance path from SCR trigger electrode 76, throughconductor 104, collector 102, and emitter 100 to ground. This lowimpedance circuit clamps the trigger electrode 76 to ground and preventsany signal passed by coupling means 28, regardless of magnitude, fromtriggering SCR 75 and providing a possibly false indication of flamepresence.

The flame switch means 30 are thus prevented from indicating flamepresence so long as the high AC voltage is present across electrodes 12and 14. When the over-voltage condition ceases or is corrected, theflame switch means will resume operation in its normal manner. An SCRmight be substituted for transistor 99 if it is desired to permanentlyclamp the input circuit to flame switch means 30 when an excessive ACvoltage is first detected, and would not release the clamp until someoperator action was taken.

It will be understood that the embodiment shown and described herein ismerely illustrative and that changes may be made without departing fromthescope of the invention as claimed.

What is claimed is:

1. In a flame monitor for determining the presence or absence of a flamewithin'a flame zone through detection of a flame generated AC signal, apair of electrode means spaced from oneanother and disposed within saidflame zone to be in contact with said flamewhen present; circuit meansoperatively-connected tosaid electrode means for applying a directcurrent potential between said'electrode means to develop a flamegenerated AC signal additionally appearing between'said electrode meanswhen a flame is present; coupling means having an input and an outputand adapted for passing to said output thereof AC signal energycharacteristic of AC signal energy applied to said input thereof; meanselectrically connecting said electrode means with said coupling meansinput for applying AC signal energy thereto; flame switch circuit meanshaving an input circuit operatively connected to the output of saidcoupling means and responsive to signal energy appearing thereat forproviding an output response commensurate with flame presence when themagnitude of said signal energy is at least as great as a certain valueand providing an output response commensurate with flame absence whenthe magnitude of said energy is less than said certain value; andclamping means op eratively connected between said electrode means andsaid flame switch circuit means and responsive to AC voltage magnitudefor effecting said flame switch circuit means output responsecommensurate with flame absence when the AC voltage across saidelectrodes exceeds a predetermined level, regardless of the signalenergy passed by said coupling means.

2. The apparatus of claim 1 wherein said clamping means include ACvoltage level detection means connected across said electrode means fordeveloping a switching voltage when AC voltage exceeding saidpredetermined level appears across said electrode means; and switchmeans responsive to the presence of said switching voltage for clampingthe input circuit of said flame switch'circuit means to a maximumelectrical energy level less than that required to provide said responsecommensurate with flame presence, whereby the output response of saidflame switch circuit means is commensurate with flame absence.

3. The apparatus of claim 2 wherein said flame generated AC signalapplied to the input of said coupling means comprises AC electricalenergy above a certain frequency and having an AC voltage substantiallyen tirely within a range less than said predetermined level and saidcoupling means include filter means operatively connected between theinput and output of said coupling means for attenuating the input signalof said coupling means to an output magnitude less than that required toprovide said response commensurate with flame presence when saidcoupling means input signal is less than said certain frequency and saidpredetermined level of AC voltage and for passing the input signal ofsaid coupling means to the output thereof at a magnitude at least asgreat as said certain value required to provide said responsecommensurate with flame presence when said AC flame signal is presentand the AC voltage at said input is below said predetermined level andwhen said AC voltage present at said means for conducting only when apotential thereacross exceeds said predetermined level of AC voltage;means for isolating said threshold discharge means from said DCpotential established between said pair of electrode means; andimpedance means for developing thereacross said switching voltage whensaid threshold discharge means conducts.

6. The apparatus of claim 5 wherein said switch means for clamping saidinput circuit of said flame switch circuit means comprise an electronicswitch having first, second, and third electrodes, said first electrodebeing operatively connected to said impedance means across which isdeveloped said switching voltage and acting to close a circuit betweensaid second and third electrodes when said switching voltage is appliedthereto and said second and third electrodes respectively beingconnected to ground potential and to the input circuit of said flameswitch circuit means.

7. The apparatus of claim 6 wherein said flame switch circuit meansinclude a load relay for providing a response commensurate with flamepresence when energized and triggerable gating means for gatingenergizing power to said load relay when triggered by a trigger signal,said gating means comprise a silicon controlled relay having an anode,cathode, and a trigger electrode and wherein energizing power is passedby circuit means from a power source to said relay by a current paththrough said anode and cathode in a triggered conductive state, saidtrigger signal being provided by that signal energy connected to theinput circuit of said flame switch circuit means and being above saidpredetermined level.

1. In a flame monitor for determining the presence or absence of a flamewithin a flame zone through detection of a flame generated AC signal, apair of electrode means spaced from one another and disposed within saidflame zone to be in contact with said flame when present; circuit meansoperatively connected to said electrode means for applying a directcurrent potential between said electrode means to develop a flamegenerated AC signal additionally appearing between said electrode meanswhen a flame is present; coupling means having an input and an outputand adapted for passing to said output thereof AC signal energycharacteristic of AC signal energy applied to said input thereof; meanselectrically connecting said electrode means with said coupling meansinput for applying AC signal energy thereto; flame switch circuit meanshaving an input circuit operatively connected to the output of saidcoupling means and responsive to signal energy appearing thereat forproviding an output response commensurate with flame presence when themagnitude of said signal energy is at least as great as a certain valueand providing an output response commensurate with flame absence whenthe magnitude of said energy is less than said certain value; andclamping means operatively connected between said electrode means andsaid flame switch circuit means and responsive to AC voltage magnitudefor effecting said flame switch circuit means output responsecommensurate with flame absence when the AC voltage across saidelectrodes exceeds a predetermined level, regardless of the signalenergy passed by said coupling means.
 2. The apparatus of claim 1wherein said clamping means include AC voltage level detection meansconnected across said electrode means for developing a switching voltagewhen AC voltage exceeding said predetermined level appears across saidelectrode means; and switch means responsive to the presence of saidswitching voltage for clamping the input circuit of said flame switchcircuit means to a maximum electrical energy level less than thatrequired to provide said response commensurate with flame presence,whereby the output response of said flame switch circuit means iscommensurate with flame absence.
 3. The apparatus of claim 2 whereinsaid flame generated AC signal applied to the input of said couplingmeans comprises AC electrical energy above a certain frequency andhaving an AC voltage substantially entirely within a range less thansaid predetermined level and said coupling means include filter meansoperatively connected between the input and output of said couplingmeans for attenuating the input signal of said coupling means to anoutput magnitude less than that required to provide said responsecommensurate with flame presence when said coupling means input signalis less than said certain frequency and said predetermined level of ACvoltage and for passing the input signal of said coupling means to theoutput thereof at a magnitude at least as great as said certain valuerequired to provide said response commensurate with flame presence whensaid AC flame signal is present and the AC voltage at said input isbelow said predetermined level and when said AC voltage present at saidinput is above said predetermined level.
 4. The apparatus of claim 3wherein said voltage level detection means include at least one gasfilled discharge device having a breakdown potential no greater thansaid predetermined level of AC voltage and resistance means fordeveloping said switching voltage, said resistance means and saiddischarge device being connected in series across said pair of electrodemeans and said switching voltage being that across said resistance meanswhen said discharge device conducts.
 5. The apparatus of claim 3 whereinsaid voltage level detection means include, in series, thresholddischarge means for conducting only when a potential thereacross exceedssaid predetermined level of AC voltage; means for isolating saidthreshold discharge means from said DC potential established betweensaid pair of electrode means; and impedance means for developingthereacross said switching voltage when said threshold discharge meansconducts.
 6. The apparatus of claim 5 wherein said switch means forclamping said input circuit of said flame switch circuit means comprisean electronic switch having first, second, and third electrodes, saidfirst electrode being operatively connected to said impedance meansacross which is developed said switching voltage and acting to close acircuit between said second and third electrodes when said switchingvoltage is applied thereto and said second and third electrodesrespectively being connected to ground potential and to the inputcircuit of said flame switch circuit means.
 7. The apparatus of claim 6wherein said flame switch circuit means include a load relay forproviding a response commensurate with flame presence when energized andtriggerable gating means for gating energizing power to said load relaywhen triggered by a trigger signal, said gating means comprise a siliconcontrolled relay having an anode, cathode, and a trigger electrode andwherein energizing power is passed by circuit means from a power sourceto said relay by a current path through said anode and cathode in atriggered conductive state, said trigger signal being provided by thatsignal energy connected to the input circuit of said flame switchcircuit means and being above said predetermined level.